The present invention relates to an apparatus for measuring the elongation of paper strips placed in an atmosphere, the humidity and temperature of which is controlled.
One of the characteristics of paper is its dilatation as a function of air humidity, at a constant temperature. To measure this characteristic, there exists apparatuses in which is disposed a series of sample strips of paper. These sample strips are subject, in an air-tight chamber, to different predetermined humidity atmospheres and their length variations are measured. The conventional apparatuses for measuring the elongation of strips comprise, for each strip, an upper and a lower gripping jaw in which the extremities of the strip are held. Each upper or lower gripping jaw is coupled to a displacement measuring instrument and to a zero setting device for the measuring instrument.
In an apparatus of the "Neenah" type available from Technidyne, measurement is made by micrometers coupled to the upper gripping jaws, and having rollers accessible from the top of the apparatus. The lower jaws are coupled to articulated levers, the horizontal position of which is visually controlled by water levels. Initially, the roller of each micrometer is activated to vertically move the associated strip until the horizontal position of the corresponding lever is obtained, which is shown by the centering of the associated level bubble. To measure the length variations, the roller of each micrometer is activated again to render the levers horizontal and the elongation is then calculated from the roller rotation.
In an apparatus of the 68936 type available from Sodexim S.A., the length is measured by comparators measuring the position of the lower gripping jaws. Initially, each comparator is reset by vertically moving the associated strip by means of a screw coupled to the upper gripping jaw and accessible from the top of the apparatus. Elongations can then be directly read on the comparators.
In the known apparatuses, it is therefore necessary, for each strip, to set the zero of an associated measurement instrument and to note down measurement, either manually (micrometer) or visually (comparator), which is tedious if there is a large number of strips. Moreover, the conventional gripping jaws have various drawbacks.
A first known type of gripping jaw is a spring clip. The spring clip can be easily handled, but does not generally enable to uniformly press paper strips. As a result, the paper strip can slide obliquely while it is being positioned or afterwards. To avoid this drawback, the spring of the clip can be stiffened, but the use of these clips is then more unpleasant, especially if there is a high number of clips, because substantial efforts are necessary to open them.
A second known more reliable type of gripping jaw comprises an eccentric pressing on a metal sheet while tightening the strip between this metal sheet and a support. However, such a gripping jaw does not allow satisfactory tightening of too thin or too thick strips. Moreover, the tightening effort is rather high and the tightening movement is not easy, which renders the use of these gripping jaws unpleasant if they are numerous.
Magnetic gripping jaws are also known, such as those described in patent DE-B-29 43 691, used to fix square sheets on a vertical plane.